Chitosan Hydrogels for Water Purification Applications

Chitosan-based hydrogels have gained significant attention for their potential applications in water treatment and purification due to their remarkable properties such as bioavailability, biocompatibility, biodegradability, environmental friendliness, high pollutants adsorption capacity, and water adsorption capacity. This article comprehensively reviews recent advances in chitosan-based hydrogel materials for water purification applications. The synthesis methods, structural properties, and water purification performance of chitosan-based hydrogels are critically analyzed. The incorporation of various nanomaterials into chitosan-based hydrogels, such as nanoparticles, graphene, and metal-organic frameworks, has been explored to enhance their performance. The mechanisms of water purification, including adsorption, filtration, and antimicrobial activity, are also discussed in detail. The potential of chitosan-based hydrogels for the removal of pollutants, such as heavy metals, organic contaminants, and microorganisms, from water sources is highlighted. Moreover, the challenges and future perspectives of chitosan-based hydrogels in water treatment and water purification applications are also illustrated. Overall, this article provides valuable insights into the current state of the art regarding chitosan-based hydrogels for water purification applications and highlights their potential for addressing global water pollution challenges.


Introduction
Life is conditioned by water for which there are no substitutes. Freshwater is a renewable natural resource, vital to the functioning of the Earth system, but present in finite quantities. Humans, plants, and animals need water to survive. Freshwater is used in agriculture, industry, power generation, sanitation, construction, transportation, and virtually in every aspect of human life. There is an imperative demand to protect freshwater ecosystems as much as possible. In addition to adapting human activities to the limits of nature's capacity, innovative techniques are needed to protect limited water resources from pollution and to ensure the preservation of an adequate supply of good quality freshwater. With the growth of the population (which has reached the threshold of 8 billion people), the evolution of modernization, and the growth of industrialization, water pollution has become an increasingly serious problem worldwide. Various harmful pollutants such as dyes, drugs, heavy metal ions, detergents, and personal care products, derivatives or by-products of various industries (e.g., oil, mining, printers, energy, etc.) are released into the environment. Figure 1 shows the most common pathways of water pollution.
If all these pollutants are discharged into aquatic ecosystems without proper treatment, they will cause water pollution [1][2][3]. A United Nations report shows that worldwide, 2 million tons of sewage and industrial and agricultural waste are discharged every day into the world's waterways and that 80% of the wastewater generated in developing countries is discharged without treatment into bodies from surface water [4]. A recent report also sounds a loud alarm, stating that water scarcity is becoming endemic as a result of the If all these pollutants are discharged into aquatic ecosystems without proper treatment, they will cause water pollution [1][2][3]. A United Nations report shows that worldwide, 2 million tons of sewage and industrial and agricultural waste are discharged every day into the world's waterways and that 80% of the wastewater generated in developing countries is discharged without treatment into bodies from surface water [4]. A recent report also sounds a loud alarm, stating that water scarcity is becoming endemic as a result of the local impact of physical water stress, along with the acceleration and spread of freshwater pollution [5]. The first consequence of the shortage is the increased use and depletion of groundwater. It is also estimated that the groundwater depletion rate is between 100 and 200 km 3 /year, representing 15 to 25% of the total groundwater withdrawals [6]. Following the UN conference, the United Nations Water Development Report 2023 highlights how building partnerships and strengthening action across all dimensions of sustainable development are essential to accelerate progress towards the Sustainable Development Goal on Clean Water and Sanitation (SDG 6) and to realize the human rights for drinking water and sanitation [7]. It was also pointed out that a growing number of chemicals threaten the aquatic environment. Once present in water, some of them manage to Several technologies have been developed for water treatment, including wastewater purification and recycling, including precipitation [10,11], oxidation [12], membrane filtration [13][14][15], adsorption [16][17][18][19][20][21][22], biological treatments [23][24][25], advanced oxidation processes [26][27][28][29][30][31][32], etc. However, some of these technologies are expensive, complex, have high energy consumption, or even low efficiency. Among them, adsorption treatment is an approach that attracts the attention of scientific research for water remediation due to several attractive properties such as simple operation, low cost, high adsorption capacity, a wide spectrum of pollutants, and favorable effects. Adsorbates such as alumina [33], clays [34,35], activated carbon [36][37][38][39][40], or biomass conversion [41] are currently used in adsorption treatment. In addition, by applying appropriate adsorption-desorption methods as well as suitable adsorbents, some pollutants, such as heavy metal ions, or critical materials (e.g., rare earth elements and lithium) can be recovered [42].
computer sciences, immunology and microbiology, and social sciences were excluded. Subsequently, to achieve the purpose of this review, the following keywords were combined: chitosan, hydrogel, water, and purification. The search was also filtered to include only reviews and articles published after 2013. Succeeding this research, the record of publications according to the category of "chitosan hydrogel for water purification" examined in this review search are smaller (161 documents) and they are presented in Figure  3c.

Adsorption Mechanisms of Chitosan-Based Hydrogels
The knowledge and evaluation of the adsorption mechanism of chitosan-based hydrogels is essential for further design and preparation of adsorption materials in order to increase the adsorption efficiency and capacity. Several adsorption mechanisms exist in the adsorption of pollutants from water using chitosan-based hydrogels such as hydrogen bonding, electrostatic interactions, ion exchanges, π-π interactions, coordination/chelation, and surface complexation (Scheme 1).

Adsorption Mechanisms of Chitosan-Based Hydrogels
The knowledge and evaluation of the adsorption mechanism of chitosan-based hydrogels is essential for further design and preparation of adsorption materials in order to increase the adsorption efficiency and capacity. Several adsorption mechanisms exist in the adsorption of pollutants from water using chitosan-based hydrogels such as hydrogen bonding, electrostatic interactions, ion exchanges, π-π interactions, coordination/chelation, and surface complexation (Scheme 1).
In Table 1, the mechanisms of adsorption, their definitions, and several examples are represented.
Due to the fact that the presence of the different functional groups on the hydrogel surface can interact physically or chemically with contaminants following one or more adsorption mechanisms, chitosan-based hydrogels are promising adsorbent materials for wastewater purification and treatment. Gels 2023, 9, x FOR PEER REVIEW 6 of 39 Scheme 1. Adsorption mechanisms of pollutants from water by chitosan-based hydrogels.
In Table 1, the mechanisms of adsorption, their definitions, and several examples are represented. high removal efficiency of methylene blue (MB) [76] electrostatic interactions the interaction between two oppositely charged molecules or/and atoms carboxymethyl cellulose/chitosan hydrogel (CMC-CS) removal of anionic and cationic dyes (MB and acid orange II) [77] ion exchanged exchange of free mobile ions of an insoluble solid with different ions of a similar charge from the solution, the reaction being reversible chitosan-gelatine ion exchanger with the addition of zirconium (IV) an increased selectivity for cationic dyes-removal of MB [78] π-π interactions noncovalent interactions by π-electron interactions between the acceptor and donor molecules chitosan/carboxymethyl cellulose hydrogel using graphene oxide as chemical crosslinking agent used to treat dyes from contaminated wastewater [79] coordination/ chelation the connection through a coordination bond of a metal ion with two or more coordination atoms (non-metal) in the same molecule to form a chelate ring structure DNA-chitosan hydrogel the removal of heavy metal ions, organic dyes, and pharmaceuticals [80] surface complexation The interaction between an electron pair donor and electron acceptor in order to form various complexes hydroxyapatite/chitosan composite (HAp-CS) the removal of Congo red (CR) [81]

Categories of Chitosan-Based Hydrogels for Water Purification Applications
Chitosan (CS), a natural polycationic polymer, synthesized from partial deacetylation of chitin, is the second most plentiful natural compound. CS is a heteropolysaccharide made from D-glucosamine and N-acetyl-d-glucosamine which are linked through β-1,4 glycosidic bonds. CS has reactive chemical functional free hydroxyl and amino groups, making it a valuable bio-compound for a wide range of applications. CS is a rigid and crystalline polymer, insoluble in water at neutral pH, but due to the protonation of free amino groups, it can be dissolved in acidic conditions (e.g., acetic acid, formic acid, and citric acid). The average molecular weight of commercially produced CS is between 3800-20,000 Daltons. In addition to its antiviral and antifungal properties, the good antimicrobial activity of CS varies depending on the degree of deacetylation, molecular weight, concentration, solution pH, viscosity, and the target microorganism [82]. Another distinctive advantage of CS is its high pollutant binding capacity. It is also biocompatible and highly abundant, biodegradable and sustainable, non-toxic, and renewable and has an adaptable surface chemistry and a high surface area, and a relatively low production cost. These beneficial characteristics make CS a versatile ecological material, usable in water purification, biofiltration, pollutant removal, and wastewater treatment [83][84][85]. CS is widely used in many other applications such as in biomedicine and pharmaceutics, agriculture, environmental sensing, cosmetics, nutritional enhancement, surface biosensor applications, fabrics and textiles, veterinary medicine, and food processing. Since it is a biocompatible, fairly inexpensive material with excellent characteristics that can be used to make hydrogels with good mechanical strength, CS-based hydrogels have a huge prospect for development. With a high degree of surface and groundwater pollution, due to dye-laden industrial wastewater, effluents pose an imminent danger to the environment and human health. A recent comprehensive review analyzed dye wastewater, stating that approximately 80% of industrial wastewater is discharged into the environment without treatment, of which 17-20% is dominated by dyes such as methylene blue (MB) and methyl orange (MO) from the textile industry. Only about 5% of textile dye is used in the dyeing process and the rest is thrown away. The study revealed that about 1.84 × 10 10 m 3 of dye wastewater is produced as waste from various industries such as tanneries, paper and pulp, dyes, textiles, and paints. Among these, the textile industry is considered the biggest water polluter, due to its approximately 100,000 types of dyes and a total annual production of 7 × 10 5 tons. The same report shows an estimate for the textile industry, which would produce about 15% of the total dye wastewater or about 1000 tons of waste annually, with a concentration of 5-1500 ppm that is discharged into aquatic waters [86]. In this context, for the protection of human health and aquatic ecosystems, the treatment of discharged effluents and compliance with environmental standards require urgent measures to be taken. Since the complex aromatic structures of dyes are quite resistant to conventional biological treatments [87], various alternative methods have been developed and applied. Among them, the adsorption of dyes from wastewater is considered a promising procedure due to its operational simplicity, avoidance of additional chemical intermediates, low price, and absence of unwanted by-products [88].
From the multitude of materials proposed as adsorbents for the removal of dyes from water, composite hydrogels based on CS have been intensively studied in various applications. The main characteristics of these composites, among which are the large surface area and abundant functional groups, are favorable to the high affinity of the dye molecules to them and lead to high adsorption efficiencies [89].
MM Perju et al. prepared three cationic composite hydrogels based on CS and poly (N-2-aminoethyl acrylamide) (PAEA) covalently crosslinked with glutaraldehyde (GA) with different molar ratios between the primary amine groups and the crosslinker (GA/NH 2 = 5.3 and 12.5), with the same total polycation concentration (CPC = 2 wt%) and two polycation molar ratios (PAEA:CS = 0.25 and 1.2), respectively. The synthesized hydrogels were tested as new adsorbents for two disazo dyes (Congo Red (CR) and Direct Blue 1) from aqueous solutions. Adsorption kinetic data were fitted by the pseudo-secondorder equation, which gave the best correlation with the experimental data, for the studied systems. It was also observed that an increase in temperature (from 4 to 40 • C) led to an increase in the color removal efficiency for both dyes [90]. A series of carboxymethyl CS/phytic acid composite hydrogels with different amounts of phytic acid were synthesized to remove methyl orange and CR dyes from an aqueous solution. The composites exhibited high stability and were formed by co-assembly and strong crosslinking interactions of carboxymethyl CS and phytic acid. The experimental test results showed that the hydrogels have a stable porous structure with multiple wrinkles on the surface due to the formation of intermolecular hydrogen bonds between CMCS and PA during the in-situ polymerization process and that they can effectively adsorb MO and CR dyes according to the pseudo-first-order and pseudo-second-order models. By changing the different adsorption factors, namely the molar ratio between the two molecules and the pH of the solution, the optimal adsorption conditions of the composite hydrogels were obtained. Carboxymethyl CS/phytic acid composite hydrogel prepared with a 3:1 molar ratio showed the highest adsorption capacity for methyl orange (13.62 mg/g) and CR (8.49 mg/g) at pH 7 and room temperature. Also, this composite showed high adsorption reusability, stability, and fine swelling capacity [91]. A composite hydrogel was synthesized by introducing CS-crosslinked polyvinyl amine into the N, N -methylenebisacrylamide crosslinked polyacrylic acid network for dye adsorption from an aqueous medium. The dual crosslinking network gave the hydrogel high mechanical properties, for a maximum Gels 2023, 9, 664 8 of 36 tensile stress of up to 1.9 MPa and a strain of 92%. The adsorption capacity of MB was up to 596.14 mg/g. The hydrogel adsorption process was consistent with quasi-second-order kinetics and the Langmuir model, which demonstrated that the dye adsorption was a monolayer chemisorption process. The synthesized hydrogel presented good reusability. The adsorption efficiency for five consecutive cycles is higher than 85%. The adsorption mechanisms for the dye's chemisorption were drawn as a monolayer, based on the wellfitted Langmuir equation, and followed a pseudo-second-order kinetic. The demonstrated good adsorption capacity is described by an adsorption mechanism based on hydrogen bonds and electrostatic interactions between the functional hydrogel's groups and dye molecules [92]. In order to strengthen the ability of a hydrogel to remove some organic pollutants by adsorption, a method was devised to obtain it by incorporating natural rectorite into the hydrogel network based on CS-g-poly (2-acrylamido-2-methyl-propane-sulfonic acid) to form a rectorite-in-polymer network structure. Rectorite is a natural clay mineral of the interstratified natural silicate type, and it is used as an ecological adsorbent of pollutants. Figure 4 schematically represents the synthesis process and the structure of the obtained composite hydrogel. The influence of increasing the number of adsorbents on dye-removal efficiency was studied (for a MB concentration of 25 mg/L and pH 10). As the adsorbent dosage increased, the adsorption amount of MB gradually decreased, and the removal rate increased significantly ( Figure 5). By introducing a lower dose of rectorite (1.2 wt%) into the hydrogel, it facilitates an improvement of its adsorption capacities towards MB in several types of aqueous media: deionized water, tap water, tap seawater, Yangtze River water, and Yellow River water ( Figure 6). By incorporating a higher dose of rectorite (15.8 wt%), an improvement in the removal rate (99.6%) for MB in real waters was obtained. The adsorbent showed high adsorption efficiency in a wide pH range (2-11) and a good degree of reusability (>4 times). It was shown that the good adsorption degree of MB on the composite hydrogel was mainly determined by the driving force of the reaction (originating from the electrostatic attraction, hydrogen bonding interaction, and chemical association of the polymer network) as well as mass transfer action (ion exchange, mainly contributed by natural rectorite) [93]. Gels 2023, 9, x FOR PEER REVIEW 9 of 39 rectorite (15.8 wt%), an improvement in the removal rate (99.6%) for MB in real waters was obtained. The adsorbent showed high adsorption efficiency in a wide pH range (2-11) and a good degree of reusability (>4 times). It was shown that the good adsorption degree of MB on the composite hydrogel was mainly determined by the driving force of the reaction (originating from the electrostatic attraction, hydrogen bonding interaction, and chemical association of the polymer network) as well as mass transfer action (ion exchange, mainly contributed by natural rectorite) [93].     A composite hydrogel as spheres prepared by encapsulating CS in an organic-inorganic iron and terephthalate ligands which develop a three-dimensional (3D) network structure was used for the adsorption of Congo red (CR) [94]. The obtained hybrid structure shows hydrolytic and good thermal stability. By varying the experimental conditions (e.g., pH, adsorbent dosage, contact time, temperature, and initial concentration), the adsorption properties were studied. The adsorption kinetics were well fitted using the pseudo-first-order model. The adsorption isotherm corresponds to the Liu model. The best adsorption performance was obtained under neutral conditions (pH 7). The adsorption capacity of Congo red has a value of 590.8 mg/g at a temperature of 298 K. The adsorption capacity of the composite hydrogel spheres was 99.97% at the adsorption equilibrium. After three adsorption cycles, the recovery was nearly 85%. The adsorption mechanism is based mainly on ionic interactions and hydrogen bonding. The authors demonstrated that the composite hydrogel spheres show valuable potential in the process of removing CR dye from aqueous media [94]. RD Hingrajiya and MP Patel incorporated mag- A composite hydrogel as spheres prepared by encapsulating CS in an organic-inorganic iron and terephthalate ligands which develop a three-dimensional (3D) network structure was used for the adsorption of Congo red (CR) [94]. The obtained hybrid structure shows hydrolytic and good thermal stability. By varying the experimental conditions (e.g., pH, adsorbent dosage, contact time, temperature, and initial concentration), the adsorption properties were studied. The adsorption kinetics were well fitted using the pseudo-firstorder model. The adsorption isotherm corresponds to the Liu model. The best adsorption performance was obtained under neutral conditions (pH 7). The adsorption capacity of Congo red has a value of 590.8 mg/g at a temperature of 298 K. The adsorption capac-ity of the composite hydrogel spheres was 99.97% at the adsorption equilibrium. After three adsorption cycles, the recovery was nearly 85%. The adsorption mechanism is based mainly on ionic interactions and hydrogen bonding. The authors demonstrated that the composite hydrogel spheres show valuable potential in the process of removing CR dye from aqueous media [94]. RD Hingrajiya and MP Patel incorporated magnetic Fe 3 O 4 in CS-grafted acrylamide and N-vinylimidazole composite hydrogels by a water-mediated free radical polymerization technique with ammonium persulfate/tetramethyl ethylenediamine as the initiator as a new type of adsorbent for MB. The prepared magnetic composite hydrogel was characterized morphologically and structurally as well as by the study of the swelling behavior. Experimental adsorption tests showed that the maximum removal of MB (q e = 860 mg/g) was obtained at pH 8, contact time 50 min, room temperature, and an initial concentration of 1000 mg/L of MB. The adsorption kinetics and adsorption isotherm models for the adsorption removal of MB on the composite hydrogel could be satisfactorily described by the second-order kinetics and the Langmuir adsorption isotherm, respectively. The composite hydrogel was proposed as a promising recyclable, durable, robust, and efficient adsorbent for the adsorption of MB from wastewater [95]. A composite hydrogel loaded with ZnO nanoparticles was prepared by grafting acrylamide over CS and gelatin polymer chains in the presence of ammonium persulfate as the initiator and maleic acid as the crosslinker. ZnO nanoparticles embedded in the CS-gelatin hydrogel were synthesized by the free radical mechanism. The nanocomposite hydrogel demonstrated a good swelling capacity, i.e., 545.81% at pH 10. The synthesized material was morphostructurally characterized. SEM images revealed a crosslinked hydrogel network with a uniform distribution of nanoparticles, and an elemental analysis indicated the composition of elements in the hydrogel. The nanocomposite hydrogel showed good photocatalytic activity towards CR (90.8%) in sunlight and can be used as a potential photocatalyst for the photodegradation of dyes in wastewater for up to four degradation cycles. The CS-gelatin nanocomposite hydrogel showed that it can be employed as an efficient photocatalyst with good degradation capacity towards CR in wastewater [96].
Recently, we used a composite hydrogel (CPX) based on CS, polyethylene glycol, and Xanthan gum for preliminary adsorption tests of some dyes from an aqueous solution (gentian violet, methyl orange, and eosin). Since, after 24 h, the results indicated a good adsorption capacity of the synthesized hydrogel, the tests were subsequently continued with the adsorption of diclofenac sodium (DS). This experiment will be described in Section 3.1.2 ( Figure 7) [97]. gelatin hydrogel were synthesized by the free radical mechanism. The nanocomposite hydrogel demonstrated a good swelling capacity, i.e., 545.81% at pH 10. The synthesized material was morpho-structurally characterized. SEM images revealed a crosslinked hydrogel network with a uniform distribution of nanoparticles, and an elemental analysis indicated the composition of elements in the hydrogel. The nanocomposite hydrogel showed good photocatalytic activity towards CR (90.8%) in sunlight and can be used as a potential photocatalyst for the photodegradation of dyes in wastewater for up to four degradation cycles. The CS-gelatin nanocomposite hydrogel showed that it can be employed as an efficient photocatalyst with good degradation capacity towards CR in wastewater [96].
Recently, we used a composite hydrogel (CPX) based on CS, polyethylene glycol, and Xanthan gum for preliminary adsorption tests of some dyes from an aqueous solution (gentian violet, methyl orange, and eosin). Since, after 24 h, the results indicated a good adsorption capacity of the synthesized hydrogel, the tests were subsequently continued with the adsorption of diclofenac sodium (DS). This experiment will be described in Sec-

CS-Based Composite Hydrogels for Pharmaceuticals Adsorption
Novel CS ionic hydrogel beads prepared by the ionic liquid impregnation of CS with tricaprylmethylammonium chloride (TCMA) were tested for the rapid adsorption of tetracycline (TC), as a model pharmaceutical compound, from an aqueous solution. Ionic liquid impregnation with TCMA greatly improved the adsorption behavior of CS toward TC, and a removal efficiency of 90% was achieved in <45 min over a wide pH range of 5-11. Increasing the amount of TCMA enhanced the adsorption efficiency of the hydrogel beads to an optimal amount (22.42 mg/g at 45 • C). The main adsorption interactions in TC adsorption on ionic beads were considered to be due to hydrogen bonds and ion-π and XH/π interactions in TC adsorption on the adsorbent surface [98]. CS/biochar hydrogel beads were explored as economical adsorbents for the removal of ciprofloxacin from aqueous solutions. Biochar was produced from the pyrolysis of yuzu pomace (pomelo peel) waste. The maximum adsorption was >76 mg/g of the adsorbent for an initial concentration of 160 mg/L ciprofloxacin. The adsorption obeyed the second-order mechanism with the main role of intra-particle diffusion and external diffusion. The adsorption capacity decreased from 34.90 mg/g to 15.77 mg/g in the presence of 0.01 N Na 3 PO 4 , unaffected by other electrolytes, such as NaCl, Na 2 SO 4 , and NaNO 3 , with the same concentration. The removal of the pollutant was achieved by a mixed mechanism, through π-π electron donor-acceptor interactions (EDAs), hydrogen bonds, and hydrophobic interactions. The adsorption decreased over time but with a slight regeneration after each adsorption cycle. The 6th adsorption was >64 ± 0.68% (25.73 mg/g), proving that the new adsorbent is an advantageous one due to its high efficiency, low cost, and ease of preparation and separation [99]. Mahmoodi et al. synthesized an interesting hydrogel adsorbent based on graphene-CS oxide and graphene-CS oxide amine, without adding chemical crosslinking agents, for the adsorption of diclofenac. Through a simple mechanical mixing method, graphene oxide-CS hydrogel (GO-CS) and amine graphene oxide-CS (AGO-CS) adsorbents were obtained. In this case, GO nanoparticles function as both the adsorbent and crosslinking agents. After a physicochemical characterization, the adsorption parameters were optimized, with the optimal mass ratio of GO to CS being 2:5, pH 5, an initial amount of 100 ppm, and a dosage of 1.5 g/L. A removal rate of approximately 90% and 97% was achieved, respectively. The results suggest that the process is of a homogeneous and monolayer type, and the data could be described by the Langmuir model [100]. HE Ali et al. prepared a nanocomposite hydrogel based on CS, polypropenoic acid, ethylenediamine (CS/PPA/EDA, with different ratios), and magnetite as a filler, using the γ-radiation technique as a crosslinking tool. The source of irradiation was gamma ( 60 Co). The prepared CS/PPA/EDA/Fe 3 O 4 -NPs nanocomposite hydrogel exhibited an excellent adsorption capacity for the removal of the hazardous and toxic basic dyes Astrazon blue (AB, 193.21 mg/g) and Lerui Acid Brilliant Blue (LABB, 51.9 mg/g) from contaminated solutions. The kinetic model of the adsorption of AB and LABB dyes on the nanocomposite hydrogel obtained was verified, and it was observed that it is consistent with a second-order reaction of an endothermic nature, and the adsorption process is homogeneous and monolayer, according to the Langmuir model. The authors found that the hydrogel composite could be regenerated while retaining its properties, recommending it for wastewater remediation applications [101]. Luo et al. synthesized a new non-toxic CS-based hydrogel using cellulose nanocrystals (CN) and carbon dots (CD). The newly developed hydrogel was tested in the detection and adsorption of tetracycline (TC) from water ( Figure 8). The 3D porous structure of the hydrogel was highly beneficial for TC adsorption and aggregation due to the natural CN backbone and the outstanding fluorescent feature of CDs, which were used as fluorescent probes ( Figure 9). Therefore, the synthesized CH had a strong adsorption capacity and a sensitive fluorescent response. The results indicated a maximum adsorption capacity of 541.3 mg/g and a detection limit of 0.12 µg/L over the linear ranges of 0.2-1.0 µg/L and 1-200 mg/L for the selective determination and sensitivity of the antibiotic. This new hydrogel, developed as a multifunctional one, may be useful in applications for both the detection and removal of antibiotics from contaminated waters [72].     In our recent work, we prepared a new composite hydrogel based on CS, polyethylene glycol, and Xanthan gum, named CPX, by a green synthesis as an economical adsorbent for diclofenac sodium (DS) in wastewater ( Figure 10) [97]. The SEM analysis revealed an advantageous two-dimensionally ordered structure, which can lead to the improvement of the swelling capacity of the hydrogel by about 83% after 360 min ( Figure 11). The swelling analysis demonstrated that it is not dependent on pH. The experiments were made at different pH values (pH 3, pH 7, and pH 9). The results showed that the swelling degree after 4 h is 83%, independent of pH values, in the limit of experimental errors [97]. The hydrogel showed an adsorption capacity (172.41 mg/g) at the highest amount of the adsorbent (200 mg) after 350 min. The adsorption kinetics followed the pseudo-first-order model, while the adsorption mechanism was explained by the Langmuir and Freundlich models. The synthesized hydrogel could be used as a promising, eco-friendly, and costeffective adsorbent for the removal of DS from wastewater [97]. In our recent work, we prepared a new composite hydrogel based on CS, polyethylene glycol, and Xanthan gum, named CPX, by a green synthesis as an economical adsorbent for diclofenac sodium (DS) in wastewater ( Figure 10) [97]. The SEM analysis revealed an advantageous two-dimensionally ordered structure, which can lead to the improvement of the swelling capacity of the hydrogel by about 83% after 360 min ( Figure  11). The swelling analysis demonstrated that it is not dependent on pH. The experiments were made at different pH values (pH 3, pH 7, and pH 9). The results showed that the swelling degree after 4 h is 83%, independent of pH values, in the limit of experimental errors [97]. The hydrogel showed an adsorption capacity (172.41 mg/g) at the highest amount of the adsorbent (200 mg) after 350 min. The adsorption kinetics followed the pseudo-first-order model, while the adsorption mechanism was explained by the Langmuir and Freundlich models. The synthesized hydrogel could be used as a promising eco-friendly, and cost-effective adsorbent for the removal of DS from wastewater [97].  J Tang et al. performed a very interesting study for the adsorption of norfloxacin (NOR), a broad-spectrum antibiotic that is used as a treatment for both humans and animals [102]. Once in the aquatic ecosystem, through various pathways, even in small amounts, NOR residues can cause the generation of antibiotic-resistant bacteria, endangering public health. For this reason, it is very important to remove NOR from the aquatic environment. Calcium alginate (CA)/CS hydrogel precursor spheres were prepared. Acidwashed spheres (CA/CS-M) demonstrated a much better adsorption of NOR (310.6 mg/g) than the precursor CA/CS spheres. Even though acid washing was expected to remove CS from CA/CS hydrogel spheres to obtain a higher specific surface area, as scanning electron microscopy and a BET assay showed, a small part of CS remained, increasing the structural stability of the material and leading to a more negatively charged surface characterized by Zeta potential. This feature is the main reason for the increase in the fast adsorption capacity of NOR. The maximum NOR adsorption capacity of CA/CS-M hydrogel spheres was 4.5 times higher than that of the untreated hydrogel spheres. It was demonstrated by pH and density functional theory calculations that NOR was adsorbed on the CA/CTS-M hydrogel spheres mainly through electrostatic interactions. The morphology of the adsorbent did not change significantly after 15 adsorption cycles, and the adsorption performance remained very good. The prepared hydrogel spheres have a great potential to be used as eco-friendly and extremely stable adsorbents for pollutants such as norfloxacin in the aqueous environment [102]. J Tang et al. performed a very interesting study for the adsorption of norfloxacin (NOR), a broad-spectrum antibiotic that is used as a treatment for both humans and animals [102]. Once in the aquatic ecosystem, through various pathways, even in small amounts, NOR residues can cause the generation of antibiotic-resistant bacteria, endangering public health. For this reason, it is very important to remove NOR from the aquatic environment. Calcium alginate (CA)/CS hydrogel precursor spheres were prepared. Acidwashed spheres (CA/CS-M) demonstrated a much better adsorption of NOR (310.6 mg/g) than the precursor CA/CS spheres. Even though acid washing was expected to remove CS from CA/CS hydrogel spheres to obtain a higher specific surface area, as scanning electron microscopy and a BET assay showed, a small part of CS remained, increasing the structural stability of the material and leading to a more negatively charged surface characterized by Zeta potential. This feature is the main reason for the increase in the fast adsorption capacity of NOR. The maximum NOR adsorption capacity of CA/CS-M hydrogel spheres was 4.5 times higher than that of the untreated hydrogel spheres. It was demonstrated by pH and density functional theory calculations that NOR was adsorbed on the CA/CTS-M hydrogel spheres mainly through electrostatic interactions. The morphology of the adsorbent did not change significantly after 15 adsorption cycles, and the adsorption performance remained very good. The prepared hydrogel spheres have a great potential to be used as eco-friendly and extremely stable adsorbents for pollutants such as norfloxacin in the aqueous environment [102].

CS-Based Composite Hydrogels as Biosorbent of Heavy and Rare Earth Hazardous Metals
Lead is known to be a toxic heavy metal that is very harmful to both humans and the aquatic environment. The development of fast, efficient, inexpensive, and environmentally friendly materials for the treatment of lead-contaminated wastewater is a challenge [103]. RM Vieira et al. investigated the adsorption and removal potential of Pb 2+ and Ni 2+ ions from aqueous solutions of two prepared hydrogels [104]. A CS-based hydrogel and a CS/acid-activated montmorillonite composite hydrogel were prepared by copolymerizing CS, acrylic acid, and N, N -methylene bisacryl amide. The adsorption capacities of the dry CS-based hydrogel for Pb 2+ and Ni 2+ ranged from 41.06 to 30.20 and from 42.38 to 36.45 mg/g in the pH range between 5.5 and 3.5. The adsorption capacities of CS/acidactivated montmorillonite composite hydrogel ranged from 35.22 to 26.11 and from 37.16 to 42.04 mg/g in the pH range between 5.5 and 3.5. The adsorption mechanism of the two pollutant ions was evaluated, and the best kinetic fit for the adsorption of Pb 2+ and Ni 2+ ions to both hydrogels was observed using the second-order nonlinear kinetic model, indicating that the limiting phase of the adsorption process involves chemical bonding through the sharing of electrons between the adsorbent and the adsorbate [104]. Metal ion-modified hydrogels have been observed to greatly improve their pollutant removal and recovery capacity compared to that of regular hydrogels. In a recent study, H Zhao et al. synthesized a hydrogel based on sodium alginate/CS/copper ion nanofibrillar cellulose (SCC-Cu) by a semi-dissolved acidification sol-gel transition method with an internal gelation method. Also, an SCC-CuMOF composite was prepared by the in situ growth of Cu-based metal-organic frameworks (MOFs) on the surface of the SCC-Cu hydrogel [105]. The obtained materials were used to remove Pb 2+ from aqueous solutions. The effects of pH, coexisting ions, contact time, initial ion concentration, as well as temperature on Pb 2+ removal were investigated. The prepared composite material showed excellent pollutant ion adsorption performance according to the pseudo-second-order model and the Langmuir model, with the maximum adsorption capacity being 531.38 mg/g. The adsorption thermodynamic study suggested that the adsorption process was spontaneous and endothermic. The authors suggested that Pb 2+ could be adsorbed on the composite material through ion exchanges and coordination interactions [105].
Among the multitude of metallic pollutants, chromium is found natively in the environment, as it is a part of the composition of natural minerals. In nature, chromium exists in two forms: trivalent and hexavalent. If Cr(III) can be assimilated as a "beneficial" ion, because it is an essential micronutrient required for the proper functioning of many biological cycles, Cr(VI) is a dangerous element extremely toxic to all biological systems, including humans and animals. Due to bioaccumulation, it can cause genetic mutations along the food chain. Therefore, the Cr content must be strictly monitored, especially in surface and groundwater and in drinking water sources. The maximum permissible limits for total Cr content recommended by the WHO are 50 µg/L, while the European Union (EU) has set a limit value of 100 µg/L for total chromium in drinking water as part of its Drinking Water Directive. For hexavalent chromium in drinking water, the indicative value established by the WHO provisionally is 0.1 µg/L, while the maximum limit for the discharge of hexavalent chromium into the aquatic environment according to the European Union is 0.5 µg/L [106]. PB Vilela developed a hydrogel synthesized by the chemical crosslinking of CS, polyacrylic acid, and N, N -methylene bisacryl amide as a cost-effective and environmentally friendly alternative adsorbent for the treatment of water and wastewater containing heavy metals [107]. It was tested in adsorption and removal studies of chromium (VI) ions contained in aqueous solutions. The maximum chromium (VI) adsorption capacities of the dry hydrogel were 73.14 and 93.03 mg/g, according to the Langmuir and Sips nonlinear isotherm models, respectively. The best fit for the adsorption of Cr(VI) ions to the CS-based hydrogel was found with the nonlinear Redlich-Peterson isotherm model. The removal percentage of chromium (VI) at pH 4.5 and the initial metal concentration of 100 mg/L was 94.72%, the adsorption mechanism of Cr(VI) being one governed by the simultaneous formation of monolayer and multilayer interactions. The best kinetic fit for the adsorption of Cr(VI) ions to a CS-based hydrogel was found with the pseudo-nth-order non-linear kinetic model [107]. S Wankar et al. fabricated a silver-CS (Ag-CS) nanocomposite hydrogel for the rapid detection and removal of hexavalent chromium from water [108]. It was prepared at a low cost by an in situ reduction of AgNO 3 to silver nanoparticles, which were embedded in a highly porous CS-based hydrogel matrix network obtained by crosslinking under ambient conditions. The low-cost hydrogel obtained was used for the rapid detection and removal of hexavalent chromium from water. This method was chosen to ensure a high stability of the AgNPs. The Ag-CS nanocomposite hydrogel acted as a colorimetric sensor for Cr(VI) with a detection limit as low as 0.1 ppb and demonstrated a fairly high removal efficiency of 83% for Cr(VI) within a rapid 30 min interval [108].
S-C Yang et al. developed inexpensive high-performance biosorbents from abundant and renewable natural polymers (cellulose and CS) for water purification and the removal of metal ions from water [109]. The researchers synthesized cellulose-CS composite hydrogels through a co-dissolution and regeneration process using a hydrate molten salt (a 60 wt% LiBr aqueous solution) as a solvent. The addition of CS increased the functionality for metal adsorption and the specific surface area and improved the mechanical strength of the composite hydrogel, compared to the pure cellulose hydrogel. The composite hydrogel with 37% cellulose and 63% CS demonstrated an adsorption capacity of 94.3 mg/g (1.49 mmol/g) towards Cu 2+ at 23 • C, pH 5, and an initial metal concentration of 1500 mg/L, which was 10 times higher than the adsorption capacity of pure cellulose hydrogel. The metal adsorption capacity of the hydrogel was pH-dependent and was stable under slightly acidic Gels 2023, 9, 664 16 of 36 conditions (pH 3-6) but significantly decreased under strongly acidic conditions (pH < 3) due to the protonation of the amino group and the dissolution of CS. From a mixture of metals in a solution, the cellulose-CS composite hydrogel showed a selective adsorption of metals in the order of Cu 2+ > Zn 2+ > Co 2+ [109]. CB Godiya et al. used CS to develop a new environmentally friendly bilayer amine group embedded microcrystalline cellulose (MCC)/CS hydrogel, synthesized by the integration of polydopamine (PDA) and polyethyleneimine (PEI) (Figure 12) [110]. The hydrogel was investigated for the reliable and efficient extraction of copper (Cu 2+ ), zinc (Zn 2+ ), and nickel ions (Ni 2+ ) in effluents. The MCC-PDA-PEI/CS-PDA-PEI hydrogel exhibited very good Cu 2+ , Zn 2+ , and Ni 2+ adsorbances of~434.8,~277.7, and~261.8 mg/g, respectively, in a single ion adsorption due to the abundant adsorption sites. The adsorption kinetics and isotherm conformed to pseudo-second-order and Langmuir models, respectively, indicating monolayer chemisorption. In a mixture of several ions in a solution, the hydrogel removes metal cations but with a slightly higher selectivity for Cu 2+ . In several adsorption/desorption cycles, the hydrogel retained >40% metal ion adsorption and desorption capacities after four consecutive cycles. Furthermore, the in situ reduction of adsorbed Cu 2+ formed a hydrogel patterned with Cu nanoparticles that showed excellent catalytic activity in the hydrogenation of 4-nitrophenol (4-NP) to 4-aminophenol. The newly fabricated hydrogel can be described as both a bioadsorbent and an efficient biocatalyst for the extraction of heavy metal ions and the catalytic hydrogenation of 4-NP in contaminated water [110]. C Li et al. obtained a CS-based fluorescent hydrogel through a three-step synthesis strategy using two dialdehydes [111]. NO2-boron-dipyrrolemethene (BODIPY) was prepared first, and then the -NO2 group was reduced to the -NH2 group. Finally, NH2-BOD-IPY was introduced into CS via the Schiff base formation reaction via bi-aldehyde. The prepared fluorescent CS hydrogels were used for the selective detection and adsorption of Hg 2+ /Hg + in aqueous media. The adsorption capacity of the fluorescent hydrogel is 121 mg/g of mercury ions, almost seven times higher compared to the unmodified hydrogel, with a detection limit of 0.3 µM. The Hg 2+ /Hg + elimination isotherm and kinetics follow the Langmuir isotherm and pseudo-second-order kinetics, respectively [111]. DC Mota Ferreira et al. successfully synthesized complex polyelectrolyte granular structures formed by the interaction of CS and carboxymethylcellulose (CMC), namely CS/CMC macro-PECs [112]. They were tested as adsorbents for six pollutants commonly found in wastewater: sunset yellow (SY), MB, CR, safranin (S), cadmium (Cd 2+ ), and lead (Pb 2+ ). The C Li et al. obtained a CS-based fluorescent hydrogel through a three-step synthesis strategy using two dialdehydes [111]. NO 2 -boron-dipyrrolemethene (BODIPY) was prepared first, and then the -NO 2 group was reduced to the -NH 2 group. Finally, NH 2 -BODIPY was introduced into CS via the Schiff base formation reaction via bi-aldehyde. The prepared fluorescent CS hydrogels were used for the selective detection and adsorption of Hg 2+ /Hg + in aqueous media. The adsorption capacity of the fluorescent hydrogel is 121 mg/g of mercury ions, almost seven times higher compared to the unmodified hydrogel, with a detection limit of 0.3 µM. The Hg 2+ /Hg + elimination isotherm and kinetics follow the Lang-muir isotherm and pseudo-second-order kinetics, respectively [111]. DC Mota Ferreira et al. successfully synthesized complex polyelectrolyte granular structures formed by the interaction of CS and carboxymethylcellulose (CMC), namely CS/CMC macro-PECs [112]. They were tested as adsorbents for six pollutants commonly found in wastewater: sunset yellow (SY), MB, CR, safranin (S), cadmium (Cd 2+ ), and lead (Pb 2+ ). The optimum adsorption pH values at 25 • C were determined to be 3.0, 11.0, 2.0, 9.0, 10.0, and 9.0 for SY, MB, CR, S, Cd 2+ , and Pb 2+ , respectively. The prepared adsorbent demonstrated an excellent adsorption capacity for the removal of SY, MB, CR, and S dyes, as well as the heavy metal cations Cd 2+ and Pb 2+ from aqueous media. Adsorption kinetics showed that the processes, depending on the type of adsorbate, followed pseudo-first-order or pseudo-second-order models. The maximum adsorption capacity of CS/CMC macro-PECs for the removal of SY, MB, CR, S, Cd 2+ , and Pb 2+ was 37.81, 36.44, 70.86, 72.50, 75.43, and 74.42 mg/g, respectively (corresponding to 98.91%, 94.71%, 85.73%, 94.66%, 98.46%, and 97.14%). Adsorbent granular structures can be regenerated after adsorption of any of the six studied pollutants and can be reused [112]. A very interesting recent study presented the adsorption of europium ions from water [113]. This rare earth ion is used in the manufacturing of superconductors, fluorescent lamps, and in various commercial uses, and radioactive isotopes of europium are employed in nuclear medicine for radiation therapy and biomedical imaging [114]. It is also involved in the manufacturing of nuclear fuels. Sometimes, relatively small amounts of liquid nuclear waste can reach the environment. The application of adsorbent materials for the treatment of liquid radioactive waste is a feasible and effective procedure due to its high efficiency, low cost, selectivity, and accessibility, adsorption being considered one of the most advantageous methods of radionuclide extraction. The paper presented the creation of a hydrogel obtained by copolymerizing acrylamide and maleic acid on the surface of CS using gamma radiation. The adsorption of Eu ions from an aqueous solution was initially fast, with the hydrogel showing a maximum adsorption capacity of 144.96 mg/g. The adsorption kinetics were described by the pseudo-second-order model, and the adsorption process was spontaneous, exothermic, and favorable at a lower temperature. An efficient desorption of Eu ions was achieved in the presence of 0.1 M HCl and AlCl 3 (97.09% and 88.63%, respectively) [113]. Table 2 summarizes the latest developments in CS-based composite adsorbent hydrogels for the removal of pollutants from wastewater.

CS-Based Beads Hydrogels
Various adsorption mechanisms (electrostatic interactions, complexation, hydrophobic interactions, coordination/chelation, pore filling, ion exchange interactions, and hydrogen bonding) between sorbent and sorbate are involved during the adsorption of different pollutants from wastewater by means of CS beads [135].
There are numerous CS beads used for the removal of pollutants from waters: CSbased composite beads, functionalized CS beads, metal-organic framework-based CS beads, magnetic CS beads, co-polymeric CS beads, and imprinted CS beads. A brief summary of the types of CS beads and their main advantages and disadvantages and the involving mechanisms are presented in Table 3. CS/Ag-hydroxyapatite nanocomposite beads ion exchange [137] CS/waste active sludge char beads chelation [138] La-Fe incorporated CS beads electrostatic interaction and ligand exchange [139] CS/lanthanum hydrogel beads electrostatic attraction and ion exchange [140] Functionalized CS beads high surface area, a multitude of available active sites, and good reusability unclear adsorption mechanism ethylenediamine tetraacetic acid grafted/polyvinyl alcohol/CS beads chemisorption [141] functional β-cyclodextrin CS beads electrostatic interaction [142] triethylenetetramine-CS/alginate composite beads electrostatic interaction, hydrogen bonding, redox reaction, chelation, and cation-anion exchange [143] novel aliquot-336 impregnated CS conjugated beads electrostatic interaction [144] CS beads impregnated hexadecyl amine chemisorption [145]  copperbenzenetricarboxylate (Cu-BTC), polyacrylonitrile (PAN), and CS physical adsorption [147] Al-MOF@CS composites with the MOFs Alfum and MIL-160 CS/PEO@MOF-5 membrane - [149] PAN/CS complexation [150] Magnetic CS beads good mechanical strength, good stability, good recovery, and higher surface area lower reusability magnetic kaolinite immobilized CS beads - [151] magnetic kaolinembedded CS beads physical adsorption [152] magnetic Fe 3 O 4 /graphene oxide/CS beads - [153] magnetic cationic polymer CS beads electrostatic interaction and ion exchange [154] Co-polymeric CS beads a good adsorption capacity lower stability PEI-grafted CS beads - [155] graft copolymerized CS with acetylacetone complexation [156] epichlorohydrin-modified CS-2,4dichlorobenzaldehyde composite homogeneous monolayer chemical adsorption [157] Zr 4+ and glutaraldehyde crosslinked PEI functionalized CS electrostatic action, complexation, and reduction [158] Imprinted CS beads good reusability and greater efficiency unclear adsorption mechanism ion-imprinted crosslinked CS chemisorption [117] imprinted CS-acrylamide hydrogen bond and electrostatic attraction [156] imprinted magnetic CS beads enthalpy controlled process [116] ion-imprinted tetraethylenepentamine modified CS beads diffusion process [159] Here, the preparation, adsorption mechanisms, and capacities of several CS-based beads' hydrogels are extensively examined. Recently, it has been shown that CS has an increased capacity to cover alginate beads with an influence on the diffusion rate of the encapsulated compounds or with the proven action of being an excellent additive with the role of modifying the bead structure [160]. Bioadhesive, biodegradable, and biofunctional alginate-CS beads were developed by an ionotropic gelation preparation method using dysprosium (Dy 3+ ) ion as the crosslinker for the effective removal of anionic Gels 2023, 9, 664 21 of 36 (Fluorescein) and cationic (Rhodamine B) dyes. Adsorption kinetic studies demonstrated that the adsorption process is controlled by a chemisorption mechanism. Physical, chemical, and rheological analyses confirmed the stable and excellent mechanical properties and demonstrated the potential of the obtained hydrogel to be used for effective wastewater treatment [160]. The newly produced amidoxime-grafted sepiolite-based CS composite beads as an organic-inorganic-hybrid system was used for the removal of dyes and toxic metals from wastewater [161]. CS-amidoxime-grafted sepiolite composite beads were thermally stable, had good mechanical properties and presented an excellent adsorption capacity (99 mg/g for copper ions). The adsorption process is governed by a homogenous mechanism with the formation of a monolayer. A novel polymeric material obtained from CuO nanoparticles synthesized by a green method from Ficus retusa plant extract and embedded in the CS bead matrix was used for the removal of CR and eriochrome black T organic dyes with an adsorption capacity of 119.7 mg/g for CR and 235.7 mg/g for eriochrome black T [162]. The results showed a 97% removal of the dyes in a time interval of 4 h by an adsorption process at equilibrium, characterized by the Freundlich isotherm model. These CS composite beads can be reused for up to five cycles.
A novel imidazolium-functionalized polysulfone/diethylenetriaminepent-acetic acid-CS bifunctional composite bead was successfully created using the phase inversion method [163]. The material was useful for the simultaneous removal of two heavy metal cations (Cr(VI) and Cu(II)) from complex wastewater. The maximum adsorption capacities obtained at pH 5 were 56.72 mg/g for Cr(VI) and 43.05 mg/g for Cu(II). The adsorbent showed good reusability in the removal process. The adsorption isotherms were based on a Sips model which is a hybrid between Freundlich and Langmuir isotherms and which demonstrated that the adsorption process of Cr(IV) and Cu(II) ions is not represented by a simple monolayer adsorption but by a heterogenous one. Removing fluoride from drinking water is a vital process. In this regard, materials with a high degree of selectivity with high porosity and an improved surface area were represented by metal organic frameworks. Organic aluminum frameworks functionalized with 2-aminobenzene-1,4-dicarboxylic acid and incorporated into CS hybrid beads were designed for water defluoridation ( Figure 13) [164]. isotherms and which demonstrated that the adsorption process of Cr(IV) and Cu(II) ions is not represented by a simple monolayer adsorption but by a heterogenous one.
Removing fluoride from drinking water is a vital process. In this regard, materials with a high degree of selectivity with high porosity and an improved surface area were represented by metal organic frameworks. Organic aluminum frameworks functionalized with 2-aminobenzene-1,4-dicarboxylic acid and incorporated into CS hybrid beads were designed for water defluoridation ( Figure 13) [164]. Figure 13. Representation of the formation mechanism of organic aluminum (Al) frameworks functionalized with 2-aminobenzene-1,4-dicarboxylic acid (ABCD) and incorporated into hybrid CS beads. Reprinted with permission from ref. [164] Copyright 2023, Elsevier.
The maximum defluoridation capacity was found to be 4.9 mg/g for a contact time of 20 min and for neutral conditions. The best fit of the fluoride adsorption model was observed for the Langmuir isotherm, leading to an adsorption mechanism based on complexation and electrostatic interactions. The obtained adsorbent can be reused for up to six cycles.
Several studies have demonstrated that CS bead-based magnetic adsorbents have improved stability and the ability to prevent the oxidation process, facilitate adsorbent Figure 13. Representation of the formation mechanism of organic aluminum (Al) frameworks functionalized with 2-aminobenzene-1,4-dicarboxylic acid (ABCD) and incorporated into hybrid CS beads. Reprinted with permission from ref. [164] Copyright 2023, Elsevier.
The maximum defluoridation capacity was found to be 4.9 mg/g for a contact time of 20 min and for neutral conditions. The best fit of the fluoride adsorption model was observed for the Langmuir isotherm, leading to an adsorption mechanism based on com- Several studies have demonstrated that CS bead-based magnetic adsorbents have improved stability and the ability to prevent the oxidation process, facilitate adsorbent separation, and lead to a total adsorption of pollutants through electrostatic interactions and complexation mechanisms [165,166]. A cellulose-CS-magnetite-alginate composite hydrogel bead was developed for the treatment of heavy metal industrial effluents [167]. The synthesis of a magnetic iron oxide tethered CS-GO nanocomposite for the removal of cationic MB dye from wastewater was reported. The maximum adsorption capacity at pH 7.4 and a contact time of 300 min was 74.93 mg/g. The adsorption kinetics fit the pseudo-second-order model, and the adsorption capacity at equilibrium was well described by the Freundlich adsorption model. The reuse was evaluated up to four successive cycles at different pH values with a good adsorption capacity [168]. Using a mixture of CS and magnetite in sodium alginate solution, a magnetic hydrogel of CS beads was obtained and used for the removal of Cu(II) ions from wastewater. At a pH value of 5 and a contact time of 24 h, the adsorbent obtained showed the maximum adsorption capacity and a removal efficiency of 56.51% [169]. Reusable and stable hydrogel-based magnetic double-network CS/CA beads were prepared and used for surfactant adsorption and MB dye removal from water [170]. The adsorption capacity of the prepared hydrogel beads (surface area of 0.65 m 2 /g) was 1.9 mg/g for the removal of cationic hexadecylpyridinium chloride and 1.2 mg/g for anionic sodium dodecyl sulfate, respectively. Adsorption kinetics followed a pseudo-second-order and Freundlich isotherm models. The adsorption process showed an efficiency of 61% in removing MB dye from an aqueous solution.
Few research studies are focused on printing technology to obtain CS-based beads. The detection of target ions using printing technologies is an efficient method due to the memory effect obtained from the preparation method. Ion-imprinted montmorillonite nanosheets/CS gel beads with a porous structure were synthesized using the ion imprinting technique for the selective adsorption of Cu(II) ions [171]. The Redlich-Peterson isotherm adsorption model was found to be suitable to describe the adsorption process. A cation exchange mechanism was found for Cu(II) ion adsorption based on SEM-EDS and XPS studies. Adsorbent performance has an efficient removal and recovery of copper ions from wastewater after five cycles. The synthesis of a new type of ion-imprinted macroporous honeycomb structured as CS-based kaolin clay composite foams using both freeze-drying and ion-imprinting methods was described by Yu et al. [172]. The synthesized ion-imprinted composite foams were used for the selective biosorption of U(VI) from wastewater. FTIR and XPS structural analyses revealed an excellent honeycomb-like structure with abundant accessibility of hydroxyl and amine functional groups. Adsorption isotherm studies indicated a maximum capacity evaluated as 286.85 mg/g for U(VI) at 298 K and pH 5, evaluated for the Langmuir isotherm model and kinetics displayed by a pseudo-second-order model. The reusability of the adsorbent is five cycles. The imprinted CS beads exhibited an outstanding adsorption capacity and superior recognition ability of the target pollutants in wastewater. Nevertheless, further research is needed to understand the interactions and preparation mechanisms for the emerging effective imprinted CS beads. Also, much attention must be paid to the development of efficient and inexpensive preparation procedures to guarantee the physical, mechanical, and chemical stability of CS beads.

CS Graft Copolymeric-Based Hydrogels
Another advantageous alternative method to functionalize the surface of natural polymers is graft copolymerization, which can be initiated by chemical methods, radiation techniques, or by other methods, such as enzymatic grafting. A molecular design specifically tailored for the targeted adsorption application can be achieved by grafting functional polymers onto the polysaccharide backbone. This is an effective method for structural modifications of hydrogels with desired functional groups and for obtaining materials with superabsorbent potential [173]. By adding functional groups such as amino, sulfur, carboxyl, and alkyl, the adsorption capacity of hydrogels can be improved. The most important parameters influencing the grafting percentage are initiator type, initiator concentration, grafting time, monomer type, monomer concentration, and temperature [174][175][176][177][178]. Other factors that can influence the properties of grafted or functionalized CS are the molecular structure, length, and number of side chains [179]. A chemical grafting requires chemical initiators (e.g., potassium persulfate, ammonium persulfate, ceric benzoyl peroxide ammonium nitrate, potassium thiocarbonate bromate, ferrous ammonium sulphate, etc.) to stimulate the polymer chains, compared to irradiation grafting which employs radiation of high energy (γ-radiation) to obtain a free radical, followed by free radical polymerization [180].
An example of a water-soluble polymer that can be easily grafted is PEI, which exhibits high hydrophilicity, long flexible chains, high charge density [181], and abundant reactive groups (-NH 2 and -NH), so that it can easily react with acid, acid chloride, epoxy, and isocyanate to modify the surface. In a recent study, C Zhang et al. successfully grafted L-cysteine (L-Cys), using PEI and CS as the carrier, obtaining two efficient, cheap, and readily available adsorbent materials chemically modified by the covalent bond of the backbone amino acids [182]. They had a good adsorption effect on Hg(II). The maximum Hg(II) adsorption capacities of PEI-Cys and CS-Cys of 231.2 mg/g and 407.4 mg/g were obtained, respectively. The results showed that L-cysteine methyl ester (an amino acid) can selectively adsorb Hg(II) in aqueous solution. The authors analyzed the adsorption behavior of Hg(II) on the two adsorbents, which can be well described by Langmuir isotherms, while the kinetics is pseudo-second order [182]. In another study, the group of NH Yusof et al. prepared CS-PEI balls by graft copolymerization under microwave irradiation and crosslinking with glutaraldehyde [183]. These were tested for the adsorption of Acid Red 27 dye from water. Crosslinking with glutaraldehyde greatly improved the mechanical and chemical properties of the obtained grafted and crosslinked CS granules (CS-PEI-GLA). Adsorption experiments were carried out in a batch system and optimized by varying the adsorbent dose, pH, initial dye concentration, and contact time. The kinetic model was described by the pseudo-second-order model, and the adsorption equilibrium was determined by the Langmuir isotherm model. The synthesized materials showed a maximum adsorption capacity of 48.3 mg/g at 27 • C. The dye adsorption mechanism was realized by electrostatic interactions [183]. For the removal of DS from water, new spherical polymeric adsorbents composed of crosslinked CS beads grafted with PEI were synthesized [184]. The crosslinking reaction improved the mechanical properties of the adsorbent, improving their handling. The presence of PEI polymer increased the number of functional groups and available adsorption sites on the surface of the adsorbent and improved the adsorption capacity for DS. The fabricated epichlorohydrin-PEI (EPCS@PEI) adsorbent demonstrated a maximum adsorption capacity of 253.32 mg/g and a removal efficiency of nearly 100% in the initial solution of 50 mg/L. The adsorption kinetics were described by the pseudo-second-order model, and the adsorption equilibrium was according to the Langmuir isotherm model. The material showed a good adsorption capacity even after recycling five times [184].

Different Composite Hydrogels Used for Water Purification
In order to highlight the importance of chitosan-based hydrogels, Table 4 shows some other different composite hydrogels based on natural biopolymers used for water purification.
Based on the data from Table 4 and our review, it can be emphasized that biopolymers, as "green" compounds from natural sources, can be used as a promising alternative for developing more sustainable materials and technologies for the removal of contaminants from wastewater and water purification.  [190] Polydopamine-montmorillonite-pullulan PDA/MMT/PUL hydrogel composites Crystal violet 112.45 [191] hydrogel nanocomposites based on methacrylated carboxymethyl cellulose (M-CMC) and lignosulfonate-derived carbonaceous products (All-lignocellulose) MB Cu (II)   350 145 [192] chondroitin sulfate/MCM-41 (chondroitin sulfate-mesoporous silica) hybrid hydrogels MB 3982 ± 123.6 [193]

CS-Based Hydrogels Used in Various Other Applications
In recent period, CS-based hydrogels have attracted tremendous interest to be used in various applications other than industrial wastewater treatment. Their applications are based on the exceptional physical, mechanical, chemical, and antibacterial properties of CS, such as mucoadhesiveness, swelling ability, low immunogenicity, enzymatic biodegradability, similarity to host tissues, excellent biocompatibility and mechanical strength, pH-sensitivity, and antibacterial effect. CS-based hydrogels are used in several other fields, including medicine and pharmaceuticals, cosmetics, the packaging and food industries, and agriculture (Scheme 2). Table 5 summarizes some examples of the recent use of CS-based hydrogels in various applications. in various applications other than industrial wastewater treatment. Their applications are based on the exceptional physical, mechanical, chemical, and antibacterial properties of CS, such as mucoadhesiveness, swelling ability, low immunogenicity, enzymatic biodegradability, similarity to host tissues, excellent biocompatibility and mechanical strength, pH-sensitivity, and antibacterial effect. CS-based hydrogels are used in several other fields, including medicine and pharmaceuticals, cosmetics, the packaging and food industries, and agriculture (Scheme 2). Scheme 2. Schematic representation of chitosan-based hydrogels in various industries. Table 5 summarizes some examples of the recent use of CS-based hydrogels in various applications.

Conclusions and Prospective Studies
The versatility of CS has broad application prospects, and CS-based hydrogels can be designed and developed with several novel functions for various high-value applications in multiple fields. The use of biodegradable, innovative, and bio-derived resources to design efficient and inexpensive adsorbent materials has increased the potential for new perspectives in wastewater remediation. CS-based hydrogels have a high capacity against various types of water pollutants due to their biodegradability, inexpensiveness, abundance, adsorption performance, and reusability. The 3D network morphology of CS-based hydrogels makes them recyclable and suitable for different viable applications. The main advantages of these types of adsorbents for use in wastewater treatment are their outstanding adsorption capacity, easy reuse and recovery, together with better mechanical, thermal, and chemical stabilities.
The present review critically examined the numerous categories of CS-based hydrogel composites and their adsorption mechanism and efficiency in removing various compounds from wastewater. The extraordinary chemical and physical ability to be used in various ways of those compounds has stimulated scientists to advance new water decontamina-tion technologies. Consequently, it has been suggested that future studies in this area should focus on emerging sustainable resources and encouraging the progress for strong, smart, biodegradable, reusable, and low-cost composites for possible further industrial, agricultural, and medicinal applications and for their commercialization.